The present invention relates to melt-processable lactide polymer compositions and processes for manufacturing such compositions from lactide.
The present disclosure concerns ongoing efforts in developing lactide polymers useable in preferred manners. U.S. Pat. No. 5,142,023 issued to Gruber et al. on Aug. 25, 1992, discloses, generally, a continuous process for the manufacture of lactide polymers with controlled optical purity from lactic acid having certain desired physical properties. A related process for generating purified lactide and creating polymers therefrom is disclosed in U.S. Pat. No. 5,338,822 issued to Gruber et al. on Aug. 16, 1994.
Generally, manufacturers of polymers utilizing processes such as those disclosed by Gruber et al. in the ""023 and ""822 patents will convert raw material monomers into polymer beads, resins or other pelletized or powdered products. The polymer in this form is typically sold to end users who extrude, blow-mold, cast films, blow films, foam, thermoform, injection-mold or fiber-spin the polymer at elevated temperatures to form useful articles. The above processes are collectively referred to herein as melt-processing. Polymers produced by processes such as those disclosed by Gruber et al. in the ""023 and ""822 patents, which are to be sold commercially as beads, resins, powders or other non-finished solid forms, are herein generally referred to collectively as polymer resins.
It is generally known that lactide polymers or poly(lactide)s are unstable. The concept of instability has both negative and positive aspects. A positive aspect is the biodegradation or other forms of degradation that occur when lactide polymers or articles manufactured from lactide polymers are discarded or composted after completing their useful life. A negative aspect of such instability is the degradation of lactide polymers during processing at elevated temperatures as, for example, during melt-processing by end-user purchases of polymer resins. Thus, the same properties that make lactide polymers desirable as replacements for non-degradable petrochemical polymers also create undesirable effects during production of lactide polymer resins and processing of these resins.
There are a number of technical problems which have heretofore inhibited development of commercially viable lactide polymer-based replacement resins for existing conventional resins. Lactide polymers are subject to unwanted degradation during melt processing via a number of pathways. These pathways include hydrolysis and other side reactions, which, for example, result in molecular weight decline and/or lactide formation. Furthermore, at high processing temperatures (especially to above about 230xc2x0 C.), lactide polymer degradation is accelerated.
Some of polylactide""s physical properties make it difficult to use for particular types of applications. In general, polylactide is a relatively brittle polymer with a low impact resistance. Because polylactide is a relatively brittle polymer, articles made of polylactide may be brittle and prone to shatter under use conditions. For example, if polylactide is used to make articles such as razor holders, shampoo bottles and plastic caps, these articles may be prone to undesirable shatter in use.
There exists a need for polylactide polymers with modified physical properties, such as impact resistance, and methods for making these types of polylactide polymers preferably without unduly compromising other physical properties, such as tensile modulus, yield strength, and blocking resistance (i.e. tendency to stick).
The invention is directed toward melt-processable polymer compositions having at least two phases. These two phases include: (a) a first phase having polylactide-based polymer; and (b) a second phase having elastomer. Each phase may be a mixture of materials. As used herein, the term xe2x80x9cmelt-processablexe2x80x9d refers to a polymer composition capable of withstanding melt-processing techniques, such as extruding, blow molding, injection molding, film casting, film blowing, foam making, thermal forming, fiber spinning, or other means used to convert the polymer at elevated temperatures to form useful articles. Preferably, the elastomer, if it is a polylactide-based polymer, is a non-thermoplastic elastomer. Herein, the term xe2x80x9cnon-thermoplastic-polylactide-based polymerxe2x80x9d is sometimes used to refer to this characteristic. If a thermoplastic elastomer is used, it can be either a polylactide-based or non-polylactide-based polymer. In many preferred applications, the elastomer is a non-polylactide-copolymerized elastomer. As used herein, the term xe2x80x9cnonpolylactide-copolymerized elastomerxe2x80x9d refers to an elastomer that does not comprise the product of a copolymerization with lactide or lactide oligomer.
The elastomer forming in second phase is preferably present in an amount effective to provide the melt-processable polymer composition with an impact resistance of at least about 0.7 ft-lb/in., when the melt-processable polymer composition has been injected molded into bars and tested according to ASTM D256 (1993) method C. Preferably, the compositions of the invention are provided with sufficient elastomer to provide an impact resistance of at least about 1 ft-lb/in. and more preferably of at least about 2 ft-lb/in., when the melt-processable polymer composition has been injected molded into bars and tested according to ASTM D256 (1993) method C. Most preferably, the elastomer is present in an amount effective to provide the melt-processable polymer composition with an impact resistance of at least about 6 ft-lb/in, when the polymer composition is tested pursuant to the Izod impact test. It is noted that, typically, most preferred compositions of the invention have an impact resistance of at least about 10 ft-lb/in when the polymer composition is tested pursuant to the notched Izod impact test.
Selected polymer compositions according to the present invention can be provided with an elongation at break of at least about 10%, a tensile modulus of at least about 200,000 psi, and a yield strength of at least about 3,000 psi, when the polymer composition has been injection molded into bars and tested according to ASTM D638-91, as described below. As discussed more fully below, in many applications, the second phase will comprise discrete domains within the first continuous phase. In others, especially those including relatively high elastomer content (i.e. about 20 wt %), the second phase may be co-continuous instead of comprising discrete domains within a continuous phase. In a composition with co-continuous phases, the first and second phases are each continuous in space and each is distributed relatively uniformly throughout the composition. Co-continuous phases are more fully disclosed in Encyclopedia of Polymer Science and Engineering, 2nd Edition, Vol. 9 at pp. 776-778 (Wiley-Intersciences, 1987). In the compositions, preferably, the first phase is the continuous phase and the second phase comprises a phase distinct from the first phase.
In a most preferred embodiment, the melt-processable composition includes a polylactide-based polymer in the first phase, and epoxidized rubber in the second phase, and a reactive compatibilizing agent. Preferably, the epoxidized rubber is present in an amount of about 1% by weight of the composition to about 40% by weight. As used herein, the weight percentages stated are the percent of the melt-processable polymer composition without regard to fillers, unless otherwise stated. More preferably, the epoxidized rubber is present in an amount of about 5% by weight of the composition to about 30% by weight. Most preferably, the epoxidized rubber is epoxidized natural rubber, present in an amount of at least about 10% by weight of the composition. Most preferably, the reactive compatibilizing agent is present in an amount of about 0.3% by weight to about 3% by weight of the composition. In another most preferred embodiment, natural rubber is combined with epoxidized rubber to form the elastomer phase. The provision of some natural rubber along with epoxidized rubbers can increase the biodegradability of the composition without substantially decreasing the resulting compositions""s relatively high impact resistance.
A wide variety of molded or blown articles, for example, dining utensils, plates, trays, disposable razor handles, and shotgun shell shot cups, can be made from the compositions of the invention. Methods of making the claimed compositions are also disclosed.
The present invention relates to polylactide-containing compositions with modified physical properties and methods for manufacturing thereof. The methods and compositions disclosed herein focus on meeting certain preferred requirements of the melt-processor and end user of the polymer resin. The improved physical properties achievable with the present invention include higher impact resistance.
Applicants have found that modifying the physical properties of polylactide polymers, such as those disclosed in U.S. Pat. Nos. 5,142,023, 5,247,058, 5,247,059, and 5,247,073, 5,258,488, 5,357,035, 5,338,822, and 5,359,026, to Gruber et al., the disclosures of which are hereby incorporated by reference, using means in accord with the invention, results in lactide polymers that can be used more effectively in a wider variety of applications and processes. The polymers of the present invention may, for example, be melt-processed into films, sheets, coatings for paper, blow molded articles, fibers, foam, foamed articles, thermoformed articles, injection molded articles (such as disposable utensils) and non-woven fabrics.
Typically, polylactide polymers, such as those disclosed in U.S. Pat. Nos. 5,142,023, 5,247,058, 5,247,059, and 5,274,073 to Gruber et al. have a relatively low impact resistance. In general, these polylactide polymers have an impact resistance of about 0.1 ft-lb/in. to about 0.4 ft-lb/in. However, for many applications, preferably polymers with an impact resistance of greater than about 0.7 ft-lbs/in., as measured by the notched Izod impact test described below, are used. Such polymers which include polylactide-based polymers and which possess many advantages concomitant therewith, are achievable with principles according to the present invention.
The notched Izod impact test is performed according to method C of ASTM method D256 (1993). A Tinius Olsen impact pendulum with either a 25 in-lb or 50 in-lb pendulum is used with standard ASTM injection molded test bars.
In general, the invention is directed toward a melt-processed polymer composition having at least two phases. The first phase contains a polylactide-based polymer. Preferably, the first phase contains a polylactide-based polymer in an amount of at least about 51% by weight of the polymer composition. A second phase of the polymer composition includes an elastomer in a sufficient amount to provide a melt-processable polymer composition having an impact resistance of at least about 0.7 ft-lb/in., when the melt-processable polymer composition has been injected molded into bars and tested according to a method C of ASTM D256 (1993). Preferably, the polymer composition also contains a compatibilizing agent, such as a reactive compatibilizing agent.
The component containing a polylactide-based polymer is combined with the elastomer containing component to form a polymer composition having at least two phases. If a compatibilizing agent is used, then it can first be combined with either the component containing the polylactide-based polymer or the component containing the elastomer. Alternatively, the compatibilizing agent can be combined simultaneously with both of these components. Applicants have found that selection of an appropriate compatibilizing agent, in particular an appropriate reactive compatibilizing agent, for use in conjunction with an appropriate elastomer component is important with respect to achieving desired levels of enhancement to the polymer""s physical properties, especially impact resistance, by the presence of the elastomer.
The present invention relates to improvements in the physical properties of polymers, especially those that contain polylactide or polylactide-based polymers. These improvements in physical properties, especially impact resistance, can be achieved, in accord with the invention, by addition of at least one phase containing elastomer. In general, Applicants have found that combining an elastomer-containing component with a component containing polylactide or containing a polylactide-based polymer, provides a melt-processable composition with desirable physical properties, such as impact resistance.
Generally, there are at least two well-known means of improving properties, such as impact resistance, of polymer compositions containing polylactide-based polymers. A first is to lower the glass transition temperature (i.e., Tg) of the polymer composition. A second involves incorporating a compound into a polylactide-based polymer in such a manner that at least one additional phase is created.
There are generally at least two well-known means for lowering the Tg of a polymer, such as a polylactide-based polymer. These means include: adding a plasticizer to the polylactide-containing resin and/or adding one or more types of copolymerizing agents to a polymerizing mixture to form a polylactide copolymer. Neither the addition of either a plasticizer or providing a copolymerizing agent (to be copolymerized with lactide) typically introduces an additional phase into the resulting polymer. Indeed, generally, the presence of a separate, elastomeric phase, would be evidenced by a second Tg at or below room temperature. Although lowering Tg is a method whereby impact strength and elongation at break can be increased, said techniques often result in a polymer having unacceptably low tensile modulus and yield strength.
In general, the present invention concerns incorporating an elastomer as an additional phase into a polylactide-containing polymer in order to improve its physical properties, especially impact resistance (or to provide a melt-processable polymer with desirable physical properties). Instead of adding a plasticizer or copolymerizing agent to the lactide polymer or prepolymer mixture, the present invention concerns mixing an elastomer with a polylactide-containing polymer to form a polymer composition having at least two phases.
In general, any polylactide-based polymer that can be combined with an elastomer to form a melt-processable polymer composition having at least two phases is usable in accord with the invention and can be improved by the processes disclosed herein. There are at least two mechanisms according to which a polylactide-based polymer can be combined with an elastomer. The first mechanism is to provide an elastomer-containing component in a prepolymer mixture containing a lactide monomer and to polymerize the lactide monomer in the presence of the elastomer to create either, a graft copolymer, or a block copolymer. To create a graft copolymer or block copolymer would typically require the elastomer-containing component to contain some functional group to facilitate reaction of the lactide monomer or polymer.
Another mechanism involves combining an already-polymerized polylactide-based polymer component with an elastomer-containing component to form a two-phase melt-processable polymer composition. In preferred applications, the claimed invention is directed toward this second mechanism of forming a two-phase polymer composition.
In accord with certain preferred applications, the first phase includes polylactide-based polymer. As used herein, the term xe2x80x9cpolylactide-based polymerxe2x80x9d can include mixtures, and refers to a polymeric component containing at least about 51% by weight polylactide or of which at least about 51% by weight is derived from lactide, lactic acid or combinations thereof, exclusive of any fillers which may be present. This polylactide-based polymer can include polylactide, poly(lactic acid), a copolymer of lactide or lactic acid, a blend of polylactide of poly(lactic acid) with other polymers, and mixtures thereof, provided that at least about 51% by weight of the copolymer and/or blend is derived from lactide or lactic acid. The first phase can include up to 100% by weight polylactide-based polymer.
Preferably, the polylactide-based polymer has a number of average molecular weight of at least about 10,000. More preferably, it has a number average molecular weight of at least about 50,000 and, most preferably at least about 80,000. Although polylactide-based polymers with a lower number average molecular weight may be used, a number average molecular weight of at least about 10,000 is preferred for obtaining a melt-processable polymer composition with desired physical properties. In addition, preferably the polylactide used is melt-stable. A melt-stable polylactide composition includes: a plurality of polylactide polymer chains preferably having a number average molecular weight of from about 10,000 to about 300,000; lactide, if present at all, present in a concentration of less than about 2% by weight; and water, if present at all, present in a concentration of less than about 1,000 parts-per-million. In a preferred polylactide composition, the concentration of lactide is less than about 0.5% by weight, and water, if present, is present in a concentration of less than about 200 parts-per-million. These weight percentages represent the percent by weight of the polylactide polymer without regard to fillers. Applicants have found that the presence of water in greater concentration can cause excessive loss of molecular weight (which affects the physical properties of the melt-processed polymer). Catalysts, water scavengers, and stabilizing agents may also be included in the polylactide composition, as disclosed in the U.S. patents to Gruber et al. reference above.
Processability in existing melt-process equipment generally requires that the molecular weight of the polylactide-based polymer be low enough so that the viscosity is sufficiently low for processing at melt-processing temperatures which do not cause substantial degradation. The lower viscosity allows for processing using existing conventional equipment. Reduced degradation prevents fouling while maintaining physical properties. This includes maintaining the initial molecular weight and viscosity through melt-processing to avoid changes in physical properties. Thus, the requirement of processability is that the polylactide-based polymer is sufficiently stable that decomposition or degradation by-products do not cause substantial fouling or plating out on existing equipment. As to physical properties of the polymer composition subsequent to melt-processing, preferably they are such that the end user or melt-processor can rely on the consistency of the polymer composition to give relatively constant tensile strength, percent elongation at break, impact strength, module of elasticity, flexural modulus, yield and flexural strength.
The polylactide-based polymer can be a copolymer resulting from polymerization of lactide with one or more additional monomers or polymers. Examples of such other monomers include glycolide, xcex5-caprolactone, any of the morpholine diones, delta-valerolactone, trimethylene carbonate, p-dioxanone, 1,5 dioxepan 2-one, 1,4 dioxepan-2-one, and/or salicylate lactones, epoxy, functional monomers, various aliphatic polyesters, and/or aliphatic polyethers.
Further, the polylactide-based polymer included in the invention can also be a copolymer resulting from polymerization of lactide with a comonomer which introduces side chains or branching or a polylactide composition which has been reacted to create branching or bridging, as disclosed in U.S. Pat. No. 5,359,026 or co-pending U.S. applications Ser. No. 08/279,732, the specifications of which are hereby incorporated by reference.
The compositions of the invention are well adapted to include polylactide-based components in the first phase that have branched and/or bridged polylactide-based compositions therein. For example, as is disclosed in the ""026 patent, a polylactide-based polymer can be formed by copolymerizing lactide, optionally other cyclic ester monomers, and a copolymerizing agent, such as an epoxidized fat or oil. These copolymerized polylactide-based polymers can be included in the first phase of the claimed invention. For example, polylactides can be copolymerized with epoxidized linseed oil and/or epoxidized soybean oil to form a polylactide-based component usable in the first phase of the invention.
In addition, polylactide molecules, which have been modified relative to linear non-substituted polylactide, to provide increased molecular interaction among polylactide backbone chains are usable in the polylactide-based component of the invention. Co-pending U.S. application Ser. No. 08/279,732 discloses polylactide compositions having improved rheology and melt-strength due to increased branching and bridging in the polymer. The polylactide-based component of the compositions of the present invention can include these rheologically improved polylactide compositions.
The polylactide-based component of the invention can also include a blend of polylactide or a polylactide copolymer with one or more additional polymers, such as polyhydroxybutyrate, polyhydroxybutyrate-co-hydroxyvalerate (PHBV), polycaprolactone, polyvinylalcohol, cellulose acetate butyrate, cellulose acetate propionate, polyesters, polycarbonates, polyamides, polydepsipeptides, polyanhydrides, polyureas, polysaccharides, and/or mixtures thereof.
The polylactide-based polymer can be the only compound included in the first phase of the invention, or the polylactide-based polymer can be combined with another compound, such as a plasticizer miscible with the polylactide-based polymer in order to form a first phase. Preferably, the plasticizer does not include substantial amounts of lower molecular weight polylactide (i.e., less than about 10,000 number average molecular weight), substantial amounts of lactide monomer, or substantial amounts of lactide oligomer. Including these types of plasticizers in substantial amounts typically decreases the melt-processability and melt-strength of the first phase component; and therefore, the final composition of the invention. Lactide monomers or oligomers are sometimes inherently present in small amounts from the polylactide processing or from depolymerization of lactide polymers. In order to facilitate the polymer composition""s melt stability, the concentrations of the lactide monomer and lactide oligomer are preferably minimized during polylactide polymer formation, as is more fully discussed in the Gruber et al. patents.
Any compounds combined with the polylactide-based polymer preferably should be ones which do not substantially inhibit or interfere with the desired physical properties to be obtained with the invention, such as a relatively high impact resistance. Further, any compounds combined with the polylactide-based polymer preferably are such as will not substantially negate the melt-processability of the composition, and should be capable of forming a relatively uniform and continuous phase with a polylactide-based polymer. Most preferably, compounds combined with the polylactide-based polymer are somewhat biodegradable. As used herein, the term biodegradable refers to chemical or biological breakdown with an ultimate mineralization by microorganism.
In a preferred embodiment, the polylactide-based polymer included in the first phase of the invention is crystallizable or is derived from a crystallizable polylactide. A reason for this preference is improved impact strength can be obtained when compared with the same polylactide-based polymer that is not crystallizable or derived from a crystallizable polylactide.
As discussed in co-pending U.S. patent application Ser. No. 08/110,394 filed Aug. 23, 1993, crystallizable polylactide, generally, is formed from a mixture containing less than about 15% by weight of meso-lactide. If the polylactide is a random polymer, then less than about 15% by weight meso-lactide is preferred, but if a block polymer is formed, the polylactide can be crystallizable even if the overall monomer composition contains more than about 15% by weight meso-lactide. In order to be crystallizable within the scope of the invention, the polylactide-based polymer or the crystallizable polylactide from which the polylactide-based polymer is derived should have the ability to crystallize when held at a particular temperature between 70xc2x0 C. and 140xc2x0 C. for the period of about one hour. After annealing at these conditions, a crystallizable polylactide-based polymer or a crystallizable polylactide from which the polylactide-based polymer is derived should have a crystallinity of at least about 10 J/g as determined by Differential Scanning Calorimetry (DSC). A temperature of about 110xc2x0 C. is often used as the temperature for crystallizing polylactide and/or polylactide-based polymers.
Preferably, the phase containing a polylactide-based polymer is at least about 51% of the polymer composition of the invention. Preferably, polylactide and/or polylactide residues make up at least about 25% by weight of the polymer composition. More preferably, at least about 50% by weight of the composition is polylactide and/or residues thereof and, most preferably, at least about 60% by weight of the composition is polylactide and/or residues thereof. The amount of polylactide and/or polylactide residues can be included in more than one phase in accord with the invention. For example, one phase of the composition may be a partially crystallized polylactide-based polymer and another phase may be a crystalline polylactide-based polymer. Preferably, the phase containing a polylactide-based polymer is a continuous phase in the composition.
In general, the macromolecular materials of interest with respect to this component can be divided into three groups, as described by W. Hofmann in section 1.3 of the Rubber Technology Handbook published by Hanser Publishers, New York (1989) and distributed by Oxford University Press. These three groups are referred to herein as plastomers, elastomers, and duromers. Plastomers, which include raw rubbers, contain no crosslinks, exhibit thermoplastic flow above the melting point, and above the melting point the elastic modulus approaches zero. Duromers are a hard rubber with a tightly crosslinked structure and a very high modulus. Elastomers have a lossely crosslinked network structure and a rubbery region above the softening point. Elastomers have a more open network structure and occupy an intermediate position between the non-crosslinked plastomer and the tightly crosslinked duromer.
An elastomer is preferably included in the invention in order to achieve improved impact resistance. Duromers are believed to have a module which is too high to be useful. Plastomers are believed by applicants to form compositions subject to blocking (undesirable sticking together of films or objects). While not being bound by theory, it is believed that the lack of crosslinks allows the plastomer phase to migrate and fuse. If antiblock agents or other measures were found to reduce blocking, then plastomers might prove acceptable. Applicants also believe that plastomers are more likely to result in a composition with undesirable anisotrophic character, as a result of the plastomer flow during final processing to form an article.
In general, elastomers can be subdivided into two groups: non-thermoplastic elastomers and thermoplastic elastomers. Non-thermoplastic elastomers maintain a useful elastic modulus up to a point of degradation. Thermoplastic elastomers maintain a useful elastic modulus up to the point at which the crosslinks melt or dissolve. Thermoplastic elastomers are elastomers which typically have crosslinks that melt or dissolve at some temperature above room temperature, but with reform upon cooling. These crosslinks are frequently crystalline segments or segments with a relatively high glass transition temperature (i.e., at last about 40xc2x0 C.). There are some thermoplastic plastomers that are capable of being converted to non-thermoplastic elastomers through crosslinks formed during processing that is required to provide the claimed compositions. This process is sometimes referred to as dynamic vulcanization. These types of thermoplastic elastomers and plastomers are also useable to provide an elastomer in the invention.
A variety of elastomers can be included in a second phase in accord with the invention to achieve improved properties, such as impact resistance, when compared to polylactide-based compositions not having an elastomeric phase. Any elastomer that is at least partially immiscible (i.e. forms a separate phase) with polylactide and/or a polylactide-based polymer is useable. It is noted that more than one type of elastomer can be included in a second phase. For example, the elastomer can be epoxidized rubber mixed with natural rubber.
The elastomer may be a thermoplastic elastomer. Some examples of these types of elastomers include those with a hard (or crystalline) segment based on a polystyrene, polyurethane, polyester, polyethylene, or polypropylene and a soft segment based on polybutadiene, polyisoprene, poly(ethylene-co-butadiene), ethylene propylene diene rubber, a polyether, a polyester (especially aliphatic polyester), a poly(ester-ether), or an aliphatic polycarbonate. The soft segment may also be a graft copolymer, block copolymer, or random copolymer, for example, of a polyester and a polyethyl or a polyester and a polycarbonate, or any mixture of the above.
The elastomer may be a non-thermoplastic elastomer, characterized by non-meltable crosslinking. Non-meltable crosslinks are crosslinks which degrade at elevated temperature and which do not reform upon cooling. Covalent bonding is an example of a typical non-meltable crosslink. The crosslinks may be present either prior to compounding or reaction with groups contained in the polylactide-based polymer, or they may be formed during the compounding or reaction with the polylactide-based polymer. Examples of useable non-thermoplastic elastomers include natural rubber and covalently cross-linked rubbers of flexible polymers and co-polymers made from monomers such as xcex5-caprolactone, p-dioxanone, lactide, glycolide, any of the morpholine diones, delta-valerolactone, 1,5 dioxepan 2-one, 1,4 dioxepan-2-one, various aliphatic polyesters, trimethylene carbonate, and mixtures thereof.
Covalently crosslinked rubbers of flexible polymers and copolymers can be made, for example, by inducing network formation during polymerization by copolymerizing monomers, such as one or more of those previously listed, with multifunctional cyclic esters or multifunctional epoxides as described in copending U.S. application Ser. No. 08/279,732, which was filed on Jul. 27, 1994 and U.S. Pat. No. 5,359,026, which issued on Oct. 25, 1994 to Gruber. The levels of addition to form a network will typically be higher than the limits set in the aforementioned application and patent, where branching without network formation was desired.
The covalently crosslinked rubbers of flexible polymers and copolymers can also be made, for example, by treatment of the corresponding plastomer with peroxide to induce a free-radical crosslinking. This can be performed during the compounding with the polylactide-based polymer.
The elastomer included in the second phase preferably has a glass transition temperature of less than about 10xc2x0 C. and more preferably less than about 0xc2x0 C.
The elastomer included in the second phase can be hydrolyzable, compostable, and biodegradable. Natural rubber and certain aliphatic polyester-based rubbers meet one or more of these criteria. The elastomer preferably is derived from annually renewable resources. Natural rubber is derived from annually renewable resources. Other modified rubbers, such as expoxidized natural rubber can also be divided (ultimately) from annually renewable resources (i.e. after modification).
Preferably, the elastomer and/or components of the compound making up a second phase contain(s) reactive functional groups which can be used to form some covalent bonding to the polylactide-based polymer included in the first phase, either directly or through use of a reactive compatibilizing agent.
Elastomers having functional groups that can react with the polylactide-based component in the composition (either directly or with a compatibilizing agent) are used according to certain preferred applications of the invention. For example, elastomers having carbonxe2x80x94carbon unsaturation, amine groups, epoxy groups, hydroxyl groups, acids, anhydrides, oxazolines, carbodiimides, isocyanates, or any other useable chain coupling agent can be used in the composition of the invention. These types of elastomers are preferred in some applications in order to facilitate bonding between the phases of the resulting composition and to provide compositions having relatively improved physical properties, such as impact resistance. Such elastomers (having functional groups) include hydroxy- or amine-containing elastomers and functionalized rubber, such as epoxidized rubbers (including, for example, epoxidized natural rubbers), maleated rubbers (including, for example, maleated natural rubbers), or acrylated rubbers (including acylated natural rubbers).
If the elastomer is combined with another compound to form the second phase of the composition, then preferably at least one compound of the second phase has reactive functional groups to form bonds with at least one component of the first phase. When polylactide is the first phase compound, then preferably the elastomer has reactive functional groups to form bonds with polylactide. The occurrence of a reaction between any of the components in the mixture can be determined by any method known to those skilled in the art. Some test methods might include analysis for reaction products by Fourier-Transform Infrared (FTIR), 13C nuclear magnetic resonance (NMR), or changes in solubility.
Applicants have discovered, as discussed below, that by combining a modified rubber, such as an epoxidized natural rubber, with a reactive compatibilizing agent and a polylactide-based polymer to form a multi-page composition, the impact resistance of resulting compositions can be maximized when compared to use of natural rubber (alone or with the same capatibilizer) or the same modified rubber, alone (i.e. without the reactive compatibilizing agent), assuming analogous mixing and processing procedures.
The amount of elastomer to be included in the composition will be varied with the end use desired of the polymer composition. In general, any amount of elastomer can be included in the elastomer containing component included in the second phase and mixed with a component containing a polylactide-based polymer to form a multi-phase polymer composition, if the desired impact resistance is achieved. However, if too much elastomer is included in the composition, then the resulting composition can have poor flow characteristics, poor tensile strength, low stiffness and low surface gloss. If an insufficient amount of elastomer is included in the composition, then the resulting composition can have insufficiently improved physical properties, such as impact resistance. Therefore, enough elastomer should be included in the composition to sufficiently improve the resulting composition""s physical properties and/or ductility but not so much that the resulting composition has undesirably low flow characteristics, poor strength, low stiffness, or low surface gloss. Preferably, the composition having more than one phase contains elastomer in an amount of at least about 1% by weight of the composition to not greater than about 40% by weight. More preferably, the composition contains elastomer in an amount of at least about 5% by weight, and in an amount of not greater than about 30% by weight. As stated previously, the amounts in percentage by weight indicate the percent of the polymer composition without regard to fillers.
In a most preferred embodiment, the elastomer comprises a mixture of epoxidized rubber and natural rubber. In this most preferred embodiment, the elastomer comprises at least about 50% by weight of the elastomer, epoxidized natural rubber.
The second phase of the invention can include elastomer in an amount of up to and including about 100% by weight of the second phase. It is preferable that the first phase having the polylactide-based polymer be a continuous phase in the composition and that the second phase containing the elastomer be a dispersed phase.
In order to assure that the multi-phase composition is as uniform as possible, any dispersed phase(s) should comprise discrete regions which are relatively small white maintaining effectiveness with respect to the desired physical properties, such as impact resistance. In a preferred embodiment, the elastomer contained in a second phase is in the form of elastomer domains or particles, which are dispersed throughout a continuous phase. Typically, for preferred applications the longest dimension of these elastomer particles is at least about 0.1 micron but not greater than about 20 microns. More preferably, the longest dimension of the elastomer particles is at least about 0.3 micron and not greater than about 5 microns. A random and substantially uniform spatial distribution of particles or domains is preferred. In addition to being in the form of a continuous phase and a dispersed phase, compositions of the invention can also include co-continuous phases. For example, an elastomer containing phase and a polylactide-based polymer containing phase can be co-continuous instead of dispersed and continuous.
In order to improve the compatibility of the phases and components contained within the phase in the polymer composition and to substantially improve the impact resistance of compositions within the scope of the invention, means of compatibilizing the phases and/or compounds are utilized in accord with certain preferred applications of the present invention. Such means of compatibilizing the phases can be either reactive, non-reactive or mixtures thereof. Adding a compound with compatibility toward the two phases in the composition can improve the polymer""s inter-phase bonding, tensile properties, impact strength resistance, and uniformity.
Selection of an appropriate compatibilizing agent is especially important to the obtaining of desired physical properties such as impact resistance. Therefore, specific preferred combinations of compatibilizing agents, elastomers and polylactide-based components to use are determined by the desired end use of the polymer compositions.
Further, a balance should be struck between the desired physical properties and the biodegradability of the resulting compositions. In many compositions of the invention, as discussed below, use of a functionalized elastomer provides a composition having a higher impact resistance than a composition in which a non-functionalized elastomer is used. However, when relatively large amounts of functionalized elastomer are included in the composition, the overall composition may not be completely biodegradable or as readily biodegradable. Further, selection of the appropriate type of compatibilizing agent is driven by the desired end use and desired end properties of the resulting composition of the invention. In addition, selection of the appropriate compatibilizing agent is driven by the particular elastomers and polylactide-based components used in the composition.
A non-reactive compatibilizing agent is one that does not provide covalent bonding with any of the components in any of the phases. A non-reactive compatibilizing agent may include a plasticizer with solubility characteristics that are between those of different phases. While not being bound by theory, it is believed that plasticizers, which can be incorporated (or partially incorporated) into the two phases, moderate the solubility characteristic, reduce the incompatibility and promote inter-phase adhesion between/among phases. Examples of plasticizers useable as non-reactive compatibilizing agents in accord with the claimed invention include plasticizers in the general classes of: alkyl esters, aliphatic esters, ethers, multi-functional esters, multifunctional ethers, and mixtures thereof. These plasticizers include alkyl phosphate esters, dialkylether diesters, dialkylether esters, tricarboxylic esters, epoxidized oils and esters, polyesters, polyglycol diesters, alkyl alkylether diesters, aliphatic diesters, alkylether monoesters, citrate ester, dicarboxylic esters, vegetable oils and their derivatives, and esters of glycerine. Most preferred plasticizers are tricarboxylic esters, citrate esters, esters of glycerine and dicarboxylic esters. These plasticizers are preferred because of their low toxicity, low volatility, compatibility with polylactide, and potential for being derived from annually renewable resources and biodegradation. Some examples of these types of plasticizers include: Citroflex(copyright) A-4, C-2, and C-4 from Morflex, Inc. of Greensboro, N.C.
A second type of non-reactive compatibilizing agent can include, for example, a block copolymer with one or more blocks which are compatible with at least one of the phases. It is believed that, for example, a di-block (A-B) copolymer, or tri-block (A-B-A) copolymer of a first segment compatible with the phase comprising elastomer and a second segment compatible with the phase comprising a polylactide-based polymer would partition itself at the interface and provide improved adhesion between the phases.
A reactive compatibilizing agent useable in accord with the claimed invention is a compatibilizing agent which forms at least some covalent bonding with at least one of the phases of the composition. For example, a compound capable of covalently bonding with the phase containing elastomer and/or the phase containing a polylactide-based polymer and which has improved compatibility with the phase with which it does not covalently bond (if any) is useable in the claimed invention. A reactive compatibilizing agent may also covalently bond with both the elastomer phase and the polylactide-based polymer phase.
Some examples of reactive compatibilizing agents useable in the claimed invention include: maleic anhydride grafted polybutadiene, epoxide functional polymers, amidated derivatives of maleic anhydride grafted polymers, hydroxy functional polymers, amine functional polymers, oxazoline functional polymers, carbiodiimine functional polymers, isocyanate functional polymers, and mixtures thereof. These polymers are preferably based on a backbone which is compatable with one of the phases in the composition, for example, on polybutadiene or polyisoprene when the elastomer phase is epoxidized natural rubber. Peroxides can also be used as reactive compatibilizing agents to promote bonding between phases. In certain preferred applications, use of a reactive compatibilizing agent provides a composition having a relatively high impact resistance.
It is believed that a catalyst can be used to facilitate reactions between any of the compatibilizing agent, polylactide-based polymer, and/or the elastomer. Suitable catalysts for reaction among particular functional groups include: phenolics, Lewis acids, and blocked Lewis acids, such as boron trifluoride monoethylamine, boron trifluoride piperidine, trimethylamine borane, and phosphorous pentafluoride complexed with amines.
Applicants have found that the selection of a reactive compatibilizing agent and a corresponding elastomer to include in a particular composition of the invention is important to obtaining a composition with most preferred characteristics. For example, Applicants have found that use of epoxidized natural rubber with a reactive compatibilizing agent provides a composition with a substantially higher impact resistance than the same composition but in which a reactive compatibilizing agent is not used. Further, use of epoxidized natural rubber (in a mixture with a polylactide-based polymer) with a reactive compatibilizing agent in accord with the invention can provide a composition with a much higher impact resistance than use of an elastomer, such as natural rubber, without a compatibilizing agent but in a mixture with the same polylactide-based polymer. In addition, because the functional groups for natural rubber are limited to Cxe2x80x94C double bonds, many reactive compatibilizing agents for use with materials containing epoxy, amine, anhydride, carbonyl, or hydroxy moieties would not be expected to be effective with it.
Use of high amounts of epoxidized natural rubber can reduce the resulting composition""s biodegradability. Polylactide-based polymers are generally biodegradable, and thus, are preferred in order to protect the environment. Therefore, in order to maintain biodegradability in a composition of the invention, the elastomer and any compatibilizing agents should also be biodegradable. In applications in which a relatively high impact resistance is not required, a biodegradable elastomer, such as natural rubber may be preferred over a less biodegradable elastomer. However, in situations where the relatively high impact resistance is desired, some reduction in biodegradability may have to be accepted for improved properties, such as increased impact resistance. In particular, epoxidized rubbers, which are less biodegradable than natural rubbers but which show much more interaction with reactive compatibilizers, may be preferred. In some instances beneficial effects can be achieved by mixing elastomers, for example, epoxidized natural rubber and natural rubber.
In accord with the invention, the reactive compatibilizing agent may be reacted during blending of the phases, or it can be reacted with one of the components prior to combining it with other components to form the composition. For example, the compatibilizing agent can be combined with the polylactide-based polymer to create a functionalized polylactide-based polymer. A functionalized polylactide-based polymer is a polylactide-based polymer which has been modified to contain groups capable of bonding to an elastomer and/or which have preferential solubility in the elastomer. Only a portion of the polylactide-based polymer may need to be functionalized in order to gain the benefit of improved impact strength, although applicants believe that a uniform distribution of functionalization throughout the polylactide based polymer would be preferred.
The functionalized polylactide-based polymer can be created during the lactide polymerization process, for example, by copolymerizing a compound containing both an epoxide ring and an unsaturated bond with lactide as is discussed in co-pending patent application Ser. No. 08/279,732. The functionalized polylactide-based polymer, containing unsaturated bonds, could then be blended and linked via free radical reactions to an elastomer with contains unsaturated bonds. A preferred embodiment of this process is more fully explained in co-pending U.S. application Ser. No. 08/279,732.
The functionalized polylactide-based polymer can also be prepared subsequent to polymerization, for example, by grafting a reactive group, such as maleic anhydride, to the polylactide-based polymer using peroxides.
The amount of compatibilizing agent used will vary with the particular end use of the polymer composition and the type of compatibilizing agent used. In general, if too much compatibilizing agent is added, physical properties of the polymer may be sacrificed or costs associated with production of the polymer may be excessive. Moreover, if too little compatibilizing agent is added the elastomer may have poor interfacial adhesion and poor ability to dissipate energy during crack propagation, which may result in decreased impact strength. Compositions of the invention having a reactive compatibilizing agent, preferably, include the reactive compatibilizing agent in an amount effective to interact with the polymer composition and increase impact resistance relative to the same composition having no reactive compatibilizing agent provided that the mixing procedures are uniform. Preferably, at least about 0.02% by weight of the composition is reactive compatibilizing agent and not greater than about 5% by weight is reactive compatibilizing agent. More preferably, at least about 0.3% by weight of the composition to about 3% by weight is reactive compatibilizing agent. If a non-reactive compatibilizing agent is used, then preferably at least about 1% by weight of the composition to about 30% by weight is compatibilizing agent. The weight % in this definition is based on the overall composition without regard to fillers.
In order to combine the components to form a mult-phase polymer composition, a high shear mixing operation can be used at an operating temperature of about 140xc2x0 C. to about 280xc2x0 C. More preferably, mixing should be done at about 150xc2x0 C. to about 200xc2x0 C. Equipment useable in the high shear mixing operation includes twin screw extruders, Banbury mixers, Buss Kneader extruder, continuous mixers, and/or single screw mixers. The components are typically combined subsequent to the polymerization process of the polylactide-based polymer.
The components included in each phase, typically, are thoroughly dried prior to being compounded, in order to minimize degradation of the polylactide-based polymer. The mixing or combining step plays an important role in determining phase domain or particle size, and mixing should be sufficiently thorough for the polymer composition to be uniform. The mixing speed and time are determined by the type of mixing/blending equipment that is used, the compounds included in the composition, and the desired end product. If the components of the composition are mixed for too long, the heat build up during mixing could facilitate undesirable degradation of any polylactide-based polymers. Further, if the components are mixed for an insufficient period of time, the resulting composition may not be uniform, the domain size may be too large, and the dispersed phase will not be evenly spatially distributed.
The size of the domains of a dispersed phase should be such that they are effective in achieving the desired physical properties. Generally, the components should be mixed until any dispersed phase(s) are in the form of domains of the desired size and are uniformly dispersed throughout the composition. As detailed in the Examples section, multiple passes of mixing on a corotating twin screw extruder have been found to be useful.
It has been found that the mixing step can be very important in determining final properties. The examples describe mixing methods which have given very positive results. While not being bound by theory, it is believed that providing a high melt viscosity throughout the mixing process is desirable. This can be achieved, to some extent, by use of high molecular weight polylactide-based polymer, temperature control of the mixing process, and staged addition of the polylactide-based polymer. It is believed that use of a semi-crystalline polylactide-based polymer feed material can also be useful and that modifying the viscosity of the polylactide based polymer, for example by use of peroxides as detailed in U.S. application Ser. No. 08/279,732, which was filed Jul. 27, 1994, can be useful. It is also noted that the use, for example, of crosslinking agents to modify the properties of the elastomer, or to convert a plastomer to an elastomer during processing, can be of benefit.
In general, a composition of the claimed invention is made by combining the polylactide-based polymer, with the non-miscible elastomer to form the multiphase composition. Preferably, the composition includes a continuous phase comprising polylactide-based polymer. Preferably, the elastomer is in a dispersed phase or co-continuous phase in an amount which is effective to provide the melt-processable polymer composition with an impact resistance of at least about 0.7 ft-lb/in. (preferably at least about 1.0 ft-lb/in. more preferably at least about 2.0 ft-lb/in. and most preferable at least about 6 ft-lb/in., when the polymer composition has been injection molded into bars and tested according to ASTM D256 (1993) method C). This amount of elastomer can vary with the type of elastomer and with the type of reactive compatibilizing agent, if any, used in the composition of the invention.
The resulting polymer composition generally has improved physical properties, such as increased ductility and impact resistance, when compared to a polylactide-based polymer composition having no elastomer-containing phase. The resulting polymer typically and preferably is one which has a number average molecular weight of at least about 50,000 and more preferably of at least about 80,000. Further, the resulting polymer composition is typically and preferably one which has a weight average molecular weight of at least about 100,000 and more preferably of at least about 160,000.
Typically and preferably, the resulting polymer compositions of the invention are ones which have an impact resistance of at least about 0.7 ft-lb/in. and preferably, an impact resistance of at least about 1 ft-lb/in., as measured using the Izod impact test previously described. More preferably, compositions of the invention are ones which have an impact resistance of at least about 2.0 ft-lb/in. and most preferably an impact resistance of at least about 6 ft-lb/in., when measured as described. It is noted that typical preferred compositions of the invention have an impact resistance of at least about 10.0 ft-lb/in when measured as described. In addition, the resulting polymer is melt-processable an can be melt-stable as described in U.S. Pat. Nos. 5,142,023, 5,247,058, 5,247,059, and 5,274,073 to Gruber et al.
The resulting polymer compositions typically have a reduced tensile modulus when compared to a polylactide polymer having no phase containing an elastomer. The tensile modulus can be measured using the test method described in ASTM D638-91 using the Type I dimensions. The crosshead speed was set at either 0.2 in/min or 2.0 in/min to obtain a break in 5 minutes or less, as described in the ASTM procedure. Typically, polylactide polymers having no phase containing an elastomer, such as those described in Gruber et al., have a tensile modulus of about 450,000 psi to about 550,000 psi. For many applications, it is desired to retain a tensile modulus of at least about 200,000 psi.
The resulting polymer compositions typically have less yield strength than comparable polylactide polymers having no elastomer-containing phase therein. The yield strength can be measured using ASTM D638-91 as discussed previously. Typically, polylactide polymers having no elastomer-containing phase therein, such as those described in the U.S. patents of Gruber et al., have a yield strength (or break strength, if they do not yield) of about 6,500 psi to about 8,500 psi. For many applications, the yield strength must be greater than some minimum value. Use of a compatibilizing agent and proper mixing can help maintain the yield strength under many circumstances. Generally, polymers in accord with the invention have a yield strength greater than about 3,000 psi.
The resulting polymer compositions, generally, have an increased percent elongation at break by comparison to the same polylactide polymer without an elastomer phase. Percent elongation at the break can be measured using the method detailed in ASTM D638-91 as discussed previously. Typically, for polylactide polymers having no elastomer-containing phase, such as those disclosed in the U.S. patents to Gruber et al., the percent elongation at break is from about 3% to about 6%. However, many applications require an improved percent elongation at break. Certain polymers of the present invention generally have a percent elongation at break greater than about 6%. Certain preferred polymers according to the present invention exhibit an elongation at break greater than at least about 10%, and in some instances, greater than about 20%. In some, the elongation at break is at least about 40%.
A melt-processable polymer composition in accord with the invention should be processable in existing melt-processing equipment by exhibiting sufficiently low viscosities at melt-processing temperatures while polymer degradation and lactide formation remain below a point of substantial degradation and do not cause excessive fouling of processing equipment. Further, the composition of the invention typically retains its molecular weight, viscosity, and other physical properties within commercially acceptable levels through the melt-processing process.
In order to further illustrate certain applications and principles of the present invention, the following examples are provided. It is noted that in these examples the notched Izod impact test results are the average of several (typically at least 5) test bars.